Modem petroleum drilling and production operations demand a great quantity of information relating to parameters and conditions downhole. Such information typically includes earth formation characteristics as well as information concerning the borehole itself. The methods that can be used to collect information downhole include: wireline logging, logging-while-drilling (“LWD”), and seismic imaging. After a well has been completed, downhole data collection may be performed by permanently emplaced sensors, by wireline logging, and by seismic monitoring.
In conventional wireline logging, a probe (“sonde”) containing formation sensors is lowered into the borehole after some or all of the well has been drilled. The sonde's upper end is attached to a conductive wireline that suspends the sonde in the borehole. The conductive wireline transports power from the surface to the sonde's instrumentation, and transports information from the sonde's instrumentation to the surface. Wireline logging generally requires that the drill string or production tubing be removed from the borehole before logging can begin. Consequently, wireline logging cannot be performed during drilling or production operations.
In LWD, as the name suggests, data may be collected during the drilling process. Collecting and processing data during the drilling process eliminates the need to remove the drilling assembly to insert a wireline logging tool. LWD consequently provides the driller with better control, allowing performance optimization and minimizing downtime. Designs for measuring downhole conditions relating to the drilling assembly's movement and location have become known as “measurement-while-drilling” techniques, or “MWD”. LWD generally concentrates more on the measurement of formation parameters, but the terms MWD and LWD often are used interchangeably. For the purposes of this disclosure, the term LWD will be used with the understanding that this term encompasses both the collection of formation parameters and the collection of information relating to the movement and position of the drilling assembly.
In LWD, the logging instrumentation is typically located at the lower end of the drill string. The instrumentation may operate, continuously or intermittently, to monitor predetermined drilling parameters and formation data. Some form of telemetry is then used to transmit the information to a surface receiver. Various telemetry systems exist, including mud pulse systems and systems that transmit acoustic signals through the drill string.
In seismic imaging, seismic waves are transmitted through earth formations and reflected from various boundaries and discontinuities. Seismic imaging involves stringing hundreds of listening devices on the surface or in a wellbore near a location where an understanding of the underground formations is desired. Once the listening devices have been placed, a disturbance is created to generate seismic waves. As these waves travel through the formations and encounter strata boundaries, some wave energy reflects back to the surface. With the appropriate processing of the received signals, a three-dimensional representation of the subsurface formations can be constructed.
The foregoing data collection methods are primarily used to locate and tap hydrocarbon reservoirs. Once a reservoir has been tapped, the goal becomes removing as much of the hydrocarbons from the reservoir as possible. Sensors may be emplaced in the wellbore to monitor pressure, temperature, fluid composition and inflow volumes. Wireline tools may be used to search for previously missed completion opportunities. Finally, long-term seismic monitoring may be employed to identify fluid migration patterns in and around the reservoir.
Information is the key to being profitable in the oil and gas industry. The more information one has regarding location and migration patterns of hydrocarbons within a hydrocarbon reservoir, the more likely it is that that reservoir can be tapped at its optimal location and utilized to its full potential. To this end, new and more sophisticated sensor arrangements are routinely created and placed downhole, so much so that existing telemetry techniques are becoming inadequate. For these reasons it would be desirable to have a communication technique that can support high speed communications between downhole sensors and a surface installation.